There are choices that need to be made if dangerous climate change is to be avoided in the 21st century. Previous reports from the Intergovernmental Panel on Climate Change (IPCC) have confirmed that the influence of human activities on the climate is unambiguous, and that climate change will have a major impact on human activities and ecosystems. Its third report, published yesterday, sets out the huge challenge that needs to be addressed by policymakers.

The evidence is clear: sticking to business as usual will lead to temperature rises of three to five degrees above pre-industrial levels. This will potentially lead to catastrophic effects on water resources and agricultural productivity, and accelerate sea level rise. If momentum is maintained, the pledges made by UN members at the Cancun Climate Convention in 2010 would probably take us to the bottom end of that range, close to a three-degree rise. But they would not meet the two-degree goal set by the UN and accepted as the target needed to avoid dangerous climate change.

Hitting the two-degree target has not been ruled out, but the IPCC has concluded in its report that major changes would be needed to energy systems, requiring technological and institutional change on a massive scale. Global emissions of greenhouse gases would need to fall by 40-70% to meet the target. This could imply a tripling or quadrupling, by 2050, of cleaner electricity sources such as renewables, nuclear or fossil fuels, along with carbon capture and storage, and the aggressive pursuit of energy efficiency opportunities. There will be no single solution across the world but rather a patchwork of responses as countries make their own choices. However, the expansion of renewable energy is likely to be ubiquitous.

The implications of delay are clearly stated in the report. If we postpone action until 2030, the two-degree target will be beyond our grasp unless technologies to remove carbon dioxide from the atmosphere are used. Afforestation and reduced deforestation will help but the alternative of burning biomass and capturing and storing the CO2 would need to be adopted. This is controversial and raises wider sustainability concerns. Prompt action will cut costs and leave open a wider range of options.

Cost is a critical question that needs to be addressed in any proposals for action, or inaction. In the report, the IPCC has suggested that action to cut greenhouse gas emissions could reduce global growth by 0.06% per year over the 21st century, leading to a 1.7% reduction in global consumption by 2030 and 3.4% by 2050 relative to a business as usual strategy. Is this affordable? It does not sound much when set against the impact of climate change. And other benefits from improved air quality and reduced dependence on imported fossil fuels are not costed in. However, expenditure would fall heavily on energy bills rather than being spread imperceptibly across the economy. The political leadership challenge is clear.

What does it mean for the UK? The Climate Change Act contains a legally binding target for an 80% reduction in greenhouse gas emissions by 2050. This is broadly compatible with the IPCC conclusion that global emissions need to fall by 40-70%. The UK also has a system of five-year carbon budgets that act as stepping stones towards the 2050 target. UK policy therefore has a built-in mechanism which, if used, can ensure that emissions reductions are not delayed. The government is considering a recommendation from the committee on climate change that the fourth carbon budget, covering 2023-27, should not be loosened. The IPCC report provides substantial support for this argument. If the UK is going to play a leading role in international negotiations, cut emissions in line with the IPCC analysis and keep its options open, the prompt action signalled by the existing fourth carbon budget is essential.

This, along with the evidence presented by the IPCC in its previous reports, should provide governments around the world with the information they need to accelerate action against the dangerous impact of climate change.